MPS Migrations Aspect

Two complementary languages upgrade user models when language definitions change:

• jetbrains.mps.lang.migration (l:90746344-04fd-4286-97d5-b46ae6a81709) — core migration language with MigrationScript (class-based, programmatic) and PureMigrationScript (declarative, structural).
• jetbrains.mps.lang.script (l:0eddeefa-c2d6-4437-bc2c-de50fd4ce470) — Enhancement Scripts that apply concept-instance transformations, used for richer per-node updates.

These address the same problem at different abstraction levels: lang.migration handles structural language refactorings (concept moves/renames, removed concepts); lang.script handles content transformations (modifying properties, children, references of existing instances).

Critical Directives

• Do not confuse the two MigrationScript concepts. One is in lang.migration (BL ClassConcept, version-gated). The other (alias "Enhancement Script") is in lang.script (instance-level transformer). Always disambiguate by language ref.
• fromVersion is the version migrated FROM, not to. Bump the language's version integer in the .mpl solution manifest whenever you add a migration step.
• Model naming is convention-driven and load-bearing. lang.migration scripts live in <language.fqn>.migration. lang.script Enhancement Scripts live in <language.fqn>.scripts. MPS discovers them by these names at startup.
• Migration scripts run in dependency order, determined by OrderDependency / ExecuteAfterDeclaration. Always declare ordering when the result of one script feeds the next.
• Do not hand-edit serialized .mps migration files. Use MPS MCP node tools.

Workflow

1. Decide form: PureMigrationScript for structural moves/renames/removals; MigrationScript for programmatic transforms; Enhancement Script for instance-level updates. See references/form-selection.md.
2. Create or locate the migration model with mps_mcp_create_model: <language.fqn>.migration for lang.migration (aspect ID migration); <language.fqn>.scripts for Enhancement Scripts (aspect ID scripts). Both aspect IDs are case-sensitive and carry no @ suffix — see aspect-model-stereotypes.md. Add the used languages required for that form.
3. Bump the language version integer in the .mpl and set fromVersion on the new script to the previous version.
4. Build the script body (declarative parts, BL execute() method, or MigrationScriptPart_Instance updater) using the JSON blueprints in references/json-blueprints.md.
5. Wire ordering (OrderDependency / ExecuteAfterDeclaration) and data flow (putData / getData) if needed.
6. Validate via mps_mcp_check_root_node_problems, then run the migration on a test model.

Related Skills

• mps-aspect-typesystem, mps-aspect-constraints — when a migration depends on or alters typesystem rules.
• mps-aspect-generator — when version changes also touch generator templates.
• mps-quotations — for QuotationConsequence consequence bodies inside MigrationScript.execute().
• mps-model-manipulation — BL + smodel constructs used inside MigrationScript.execute() and Enhancement Script updaters.

Reference Index

• Form selection guide — open before authoring to pick between PureMigrationScript, MigrationScript, and Enhancement Script for the change at hand. Covers ordering, data flow, and model setup. See references/form-selection.md.
• lang.migration concept reference — open when you need exact members, cardinalities, properties, or concept IDs for MigrationScript, PureMigrationScript, IMigrationUnit, ordering and data-exchange concepts, and RefactoringLog. See references/lang-migration-reference.md.
• PureMigrationPart, node references, and move specializations — open when defining concept/link/property renames or composing migration units via IncludeMigrationPart. See references/pure-migration-parts.md.
• MigrationScript execute-body constructs — open when writing the BL body of a programmatic MigrationScript: pattern variable references, ConsequenceFunction vs QuotationConsequence, ConceptMigrationReference. See references/migration-script-body.md.
• lang.script Enhancement Scripts — open when iterating over concept instances to update properties/children/references, or when wiring ExtractInterfaceMigration, FactoryMigrationScriptPart, or CommentMigrationScriptPart. See references/enhancement-scripts.md.
• Verified JSON blueprints — open when constructing a real migration as JSON for MCP tools. Includes MigrateReferences, SNodeTypeCastExpression_completelyRemoved, and Migrate_ParametrizedIntentions. See references/json-blueprints.md.
• Concept ID quick reference — open when assembling JSON that requires conceptReference strings for lang.migration and lang.script. See references/concept-ids.md.
• FIND_INSTANCES recipes — open to enumerate existing migration scripts or enhancement scripts in the project. See references/find-instances.md.